1. Field of the Invention
The present invention relates to an infrared camera with offset compensation.
2. Description of the Related Art
FIG. 9 is a block diagram relating to a conventional infrared camera. The drawing shows an subject M, an infrared optical system 1, an image pickup element formed on the imaging surface of the infrared optical system 1, an element temperature monitor 3 thermally connected to the image pickup element 2, a shutter 4 provided between the infrared optical system 1 and the image pickup element 2, a bias power source 5 connected to the image pickup element 2, a driver circuit 6 connected to the image pickup element 2, pre-positioned amplifying circuit 7 connected to the image pickup element 2, a display processing circuit 8 connected to the pre-positioned amplifying circuit 7, an element temperature stabilizing means 9 thermally connected to the image pickup element 2, a timing generation circuit 10 connected to the shutter 4, the driver circuit 6, and the display processing circuit 8, a body 11, an offset compensation execution switch 12 provided in the outside of the body 11 and connected to the timing generation circuit 10. The display processing circuit 8 comprises an A/D converter 13, an addition averaging circuit 14, a frame memory 15, a subtraction circuit 16, and a D/A converter 17. Also shown in FIG. 1 are a reference voltage power source 18, a differential amplifying circuit 19 connected to the element temperature monitor 3 and the reference voltage power source 18, a vacant element package 20 accommodating the image pickup element 2, the element temperature monitor 3, and the element temperature stabilizing means 9, and an infrared window 21 typically made of germanium, leaving a vacant space enclosed by the vacant element package 20 and the infrared window 21.
FIG. 10 shows an example structure of the image pickup element 2 which is, for the sake of brevity of explanation, comprised of 3xc3x973 elements. The drawing shows infrared detectors 22 to 30, transistors 31 to 48, capacitors 49 to 51, a vertical scanning circuit 52, and a horizontal scanning circuit 53. The infrared detectors 22 through 30 are microbolometers having a hollow structure, as described in Japanese Patent Laid-open No. Hei 7-509057, which are made of vanadium oxide or titanic oxide for reducing thermal conductance with respect to the surrounding so that heat quantity of absorbed infrared can be efficiently converted into an increase of temperature of the detector thereby achieving high sensitivity.
In operation, the reference voltage power source 18 outputs a reference voltage corresponding to an operation temperature of the image pickup element 2 to the differential amplifying circuit 19. The differential amplifying circuit 19 compares the supplied output and an output from the element temperature monitor 3 to feed back a power corresponding to the difference between the outputs to the element temperature stabilizing means 9 for stabilizing the operation temperature of the image pickup element 2.
Next, the bias power source 5 supplies bias voltage Vb and gate voltage Vg to the transistors 46 through 48 and the driver circuit 6 sends a driving clock to the vertical scanning circuit 52 according to a timing generated by the timing generation circuit 10 for selection of a row of infrared detectors. In response to the clock, the vertical scanning circuit 52 renders the transistors 31 through 33 conductive for a predetermined period, whereby a bias current defined according to gate voltage Vg is caused to flow into the infrared detectors 22 to 24, so that the voltage corresponding to the respective resistance values will be caused at the infrared detectors 22 through 24.
Subsequently, when a sample-hold clock is applied, the transistors 40 to 42 are made conductive so that the voltage according to the resistance values of the infrared detectors 22 to 24 is temporarily stored in the capacitors 49 to 51. Then, after shutting off the transistors 40 to 42, the horizontal scanning circuit 53 sequentially makes the transistors 43 through 45 conductive so as to output voltage according to the resistance values of the infrared detectors 22 through 24.
Thereafter, the vertical scanning circuit 52 selects the row of infrared detectors 25 through 27 so that the voltage corresponding to the resistance values thereof will be output in the same procedure as that is applied to the infrared detectors 22 through 24.
While repeating the above procedure, voltages corresponding to the resistance values of the infrared detectors 22 through 30 which constitute the image pickup element 2 are sequentially output and, after being amplified in the pre-positioned amplifying circuit 7, are supplied to the A/D converter circuit 13. Then, the timing generation circuit 10 sends a signal for closing the shutter so that the shutter is closed.
After the shutter was closed and consistent infrared were introduced into the infrared detectors 22 through 30, the timing generation circuit 10 sends a signal to the display processing circuit 8, for obtaining offset compensation data for the first time. Then, the A/D converter circuit 13 converts an output from the pre-positioned amplifying circuit 7 into a digital signal. Further, an addition average is obtained for every infrared detector in the addition averaging circuit 14 so that variation of the resistance values of the infrared detectors 22 through 30, in other words, offset variation, is stored in the frame memory 15.
Then, the shutter 4 is opened, and infrared radiation emitting from the direction of subject M is collected in the infrared optical system 1. The converged infrared radiation then passes through the infrared window 21 to form an image on the infrared detectors 22 through 30. This causes a slight increase of the temperatures of the infrared detectors 22 through 30 by an order of a few mK according to the strength of the collected infrared radiation. As a result, the respective resistance values of the infrared detectors are changed from those before the shutter 4 was opened.
Outputs from the infrared detectors 22 through 30 are then amplified in the pre-positioned amplifying circuit 7 and converted into digital signals in the A/D converter circuit 13, similar to when offset compensation data is obtained. Then, the data stored in the frame memory 15 is subtracted from the digital signals for every pixel in the subtraction circuit 16 to remove fixed pattern noise due to offset variation of the infrared detectors, and the result is converted into an analogue video signal in the D/A converting circuit 17 before being output.
Here, a change in the inside temperature of the body 11 due to heat generation of an electric circuit or a change of ambient temperature may change an output voltage of the reference voltage power source 18, characteristics of the element temperature stabilizing means 9, the amount of heat discharged from the image pickup element 2, or the amount of infrared radiation from the infrared optical system 1, resulting in a slight change to an operation temperature of the image pickup element 2. Accordingly, the resistant values of the infrared detectors 22 through 30 are changed for every pixel. Because the amount of change of the resistance value differs for every pixel, offset variation of an output is changed from that at the time when offset compensation data was first obtained, leaving outstanding fixed pattern noise in a video signal. In such a case, offset compensation data is obtained again by operating the offset compensation switch 12 to restore the image.
While the compensation operation described above as being applied to a non-cooling type of infrared camera whose image pickup element 2 has a two-dimensional array of microbolometers and operates as stabilized at a constant temperature around a room temperature, the operation may be similarly applied to a cooled infrared camera whose image pickup element 2 has a two-dimensional array of, for example, platinum and silicon Schottky barrier diodes, and operates as stabilized at a low temperature, including a typical value of around 77 K.
The structure of infrared cameras equipped as described above requires a manual switching operation for offset compensation. Therefore, an operator must stay near the camera, even for a long-time continuous use of the camera.
The present invention has been conceived to overcome the above problems and aims to provide an infrared camera which can automatically perform offset compensation to remove fixed pattern noise without requiring input or operation.
An infrared camera of the present invention comprises an offset compensation execution signal generation circuit for automatically generating an offset compensation execution signal, and a shutter arranged at a position covering the viewing field of the image pickup element.
With this arrangement, the shutter is closed according to an offset compensation execution signal automatically and periodically generated by the offset compensation signal generation circuit before carrying out offset compensation. This enables automatic offset operation without requiring an operator to manipulate the camera.
An infrared camera of the present invention may also comprise an offset compensation execution signal generation circuit for automatically generating an offset compensation execution signal, and a de-focus motor associated with an infrared optical system.
With this arrangement, the de-focus motor is operated based on an offset compensation execution signal which is automatically and periodically generated by the offset compensation signal generation circuit, to move the focusing plane of the infrared optical system for carrying out offset compensation. This enables automatic offset operation without requiring an operator.
An infrared camera of the present invention may further comprise an offset compensation execution signal generation circuit for outputting an offset compensation execution signal at a constant interval.
Further, an infrared camera of the present invention may comprise an offset compensation execution signal generation circuit for outputting an offset compensation execution signal in a shorter interval than the above mentioned constant interval during a predetermined period immediately after turning on the power.
With this arrangement, there can be provided an infrared camera which can produce a preferable image, even immediately after being powered up.
According to an infrared camera of the present invention, the offset compensation execution signal generation circuit may have a temperature sensor for measuring temperature around the structural elements of the infrared camera, and a temperature changing amount judging circuit connected to the temperature sensor.
With this arrangement, an offset compensation is applied when the temperature around the structural elements of the infrared camera has been changed by more than a predetermined value from the temperature at the time of previous execution of offset compensation. This enables production of an infrared camera which can produce a preferable image even during a period immediately after having turned on the power or when the temperature around the structural elements of the camera is changed while capturing an image.